Tartaric acid is a white, crystalline organic acid that occurs naturally in many fruits, most notably in grapes, but also in bananas, tamarinds, and citrus. Its salt, potassium bitartrate, commonly known as cream of tartar, develops naturally in the process of fermentation. It is commonly mixed with sodium bicarbonate and is sold as baking powder used as a leavening agent in food preparation. The acid itself is added to foods as an antioxidant E334 and to impart its distinctive sour taste. Naturally occurring tartaric acid is a useful raw material in organic chemical synthesis. Tartaric acid, an alpha-hydroxy-carboxylic acid, is diprotic and aldaric in acid characteristics, and is a dihydroxyl derivative of succinic acid.
History
Tartaric acid has been known to winemakers for centuries. However, the chemical process for extraction was developed in 1769 by the Swedish chemist Carl Wilhelm Scheele.
Tartaric acid played an important role in the discovery of chemical chirality. This property of tartaric acid was first observed in 1832 by Jean Baptiste Biot, who observed its ability to rotate polarized light. Louis Pasteur continued this research in 1847 by investigating the shapes of sodium ammonium tartrate crystals, which he found to be chiral. By manually sorting the differently shaped crystals, Pasteur was the first to produce a pure sample of levotartaric acid.
Stereochemistry
Naturally occurring form of the acid is dextro tartaric acid or L-(+)-tartaric acid (obsolete name d-tartaric acid). Because it is available naturally, it is cheaper than its enantiomer and the meso isomer. The dextro and levo prefixes are archaic terms. Modern textbooks refer to the natural form as (2R,3R)-tartaric acid (L-(+)-tartaric acid), and its enantiomer as (2S,3S)-tartaric acid (D-(-)-tartaric acid). The meso diastereomer is referred to as (2R,3S)-tartaric acid or (2S,3R)-tartaric acid.
Dextro and levo form monoclinic sphenoidal crystals and orthorhombic crystals. Racemic tartaric acid forms monoclinic and triclinic crystals (space group P). Anhydrous meso tartaric acid form two anhydrous polymorphs: triclinic and orthorhombic. Monohydrated meso tartaric acid crystallizes as monoclinic and triclinic polymorphys depending on the temperature at which crystallization from aqueous solution occurs. Tartaric acid in Fehling's solution binds to copper(II) ions, preventing the formation of insoluble hydroxide salts.
Production
L-(+)-Tartaric acid
The L-(+)-tartaric acid isomer of tartaric acid is industrially produced in the largest amounts. It is obtained from lees, a solid byproduct of fermentations. The former byproducts mostly consist of potassium bitartrate (KHC4H4O6). This potassium salt is converted to calcium tartrate (CaC4H4O6) upon treatment with calcium hydroxide "milk of lime" (Ca(OH)2): KH(C4H4O6) + Ca(OH)2 -> Ca(C4H4O6) + KOH + H2O In practice, higher yields of calcium tartrate are obtained with the addition of calcium chloride. Calcium tartrate is then converted to tartaric acid by treating the salt with aqueous sulfuric acid: Ca(C4H4O6) + H2SO4 -> H2(C4H4O6) + CaSO4
Racemic tartaric acid
Racemic tartaric acid can be prepared in a multistep reaction from maleic acid. In the first step, the maleic acid is epoxidized by hydrogen peroxide using potassium tungstate as a catalyst. HO2CC2H2CO2H + H2O2 → OC2H2(CO2H) 2 In the next step, the epoxide is hydrolyzed. OC2H2(CO2H)2 + H2O → (HOCH)2(CO2H)2
meso-Tartaric acid
A mixture of racemic acid and meso-tartaric acid is formed when dextro-Tartaric acid is heated in water at 165 °C for about 2 days. meso-Tartaric acid can also be prepared from dibromosuccinic acid using silver hydroxide: HO2CCHBrCHBrCO2H + 2 AgOH → HO2CCH(OH)CH(OH)CO2H + 2 AgBr meso-Tartaric acid can be separated from residual racemic acid by crystallization, the racemate being less soluble.
Reactivity
L-(+)-tartaric acid, can participate in several reactions. As shown the reaction scheme below, dihydroxymaleic acid is produced upon treatment of L-(+)-tartaric acid with hydrogen peroxide in the presence of a ferrous salt.
HO2CCH(OH)CH(OH)CO2H + H2O2 → HO2CC(OH)C(OH)CO2H + 2 H2O
Dihydroxymaleic acid can then be oxidized to tartronic acid with nitric acid.
Derivatives
Important derivatives of tartaric acid include: Sodium ammonium tartrate, the first material separated into its enantiomers cream of tartar (potassium bitartrate), used in cooking Rochelle salt (potassium sodium tartrate), which has unusual optical properties tartar emetic (antimony potassium tartrate), a resolving agent. Diisopropyl tartrate is used as a co-catalyst in asymmetric synthesis.